The name for this particular practice has come to stand for more stringent kashrut standards in general.
Lungs are a special concern
An exception is the lung of an animal, on which adhesions [sirhot] and other problems may develop. While these problems are not common, they do occur more frequently than other trefot [i.e., defects]. Their relative prevalence led the rabbis to mandate that the lungs of every animal be examined, both manually while still in its natural position in the animal, and visually following its removal from the thoracic cavity (Shulchan Arukh, YD 39:1). (Nowadays, another problem that occurs with relative frequency and is therefore also inspected for is holes of the second stomach, the bet ha-kosot [reticulum], caused by animals eating nails and other sharp metal objects.) Because a hole in the lung renders the animal a trefa, adhesions, i.e., pathologically arising bands of collagen fibers, are problematic either because they indicate the presence of a perforation that has been insufficiently sealed ([so says Talmud commentator] Rashi) or because they can become loosened, thereby causing a hole to develop ([so say the Talmud commentators known collectively as] Tosafot).
In the U.S., lung adhesions usually do not occur on fowl; hence the rest of this discussion concerns only meat, not chicken. (The lungs of fowl can have defects that render it trefa, but not the same kind of adhesions that occur in animals. There are those who feel that nowadays fowl lung problems are also becoming more prevalent and thus require a visual and tactile inspection of fowl lungs.)
The Shulchan Arukh describes many types of adhesions in intricate detail (YD 39:4-13), the overwhelming majority of which render the animal a trefa. [Rabbi Moshe Isserles, the sixteenth century Polish rabbi whose comments on Shulchan Arukh are incorporated into every edition] (YD 39:13) concludes the discussion about lung adhesions with a description of a method of peeling and testing many types of adhesions, thereby resulting in many more animals determined to be kosher. [Isserles] himself expressed certain hesitations about aspects of this leniency, but because it had gained wide acceptance and did have a firm basis, he ruled that it could be followed. However, he cautions that the peeling and testing must be performed by an exceedingly God-fearing individual.
Glatt in Our Time
Because this peeling is mentioned and approved by [Isserles] but not by the author of the Shulhan Arukh, Rabbi Yosef Karo, Sephardim, who follow the rulings of Rabbi Karo, are required to eat only glatt (halak, in Hebrew) meat as defined by Karo. [Karo] is also the author of the Bet Yosef [an extensive commentary to an earlier code of Jewish law]; therefore, such meat is termed "glatt/halak Bet Yosef." For Ashkenazim, there is a tradition that a small, easily removable adhesion is defined as a lower class of adhesion, known as rir, and that the presence of up to two such small, easily removable adhesions still qualifies the animal as glatt according to Ashkenazic tradition. Eating glatt is a worthy stringency that avoids potential problems raised by [Isserles's] controversial leniency.
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